Inadequate methods and compositions to effectively heal chronic wounds is a significant health care problem. Impaired wound healing increases the chances of mortality and morbidity. This problem is especially prominent in patients with diabetes who develop severe, life threatening wounds on body extremities. Chronic diabetic foot ulcers often lead to amputations. These wounds are often the result of poor circulation derived from the diabetic patients' insulin-compromised cells as well as impaired vascularization of the wound bed, reduced infiltration of germ fighting cells and reduced tissue epithelialization. As a result, most current therapies include attempts to revascularize the wound bed and prevent infection.
Wounds in non-compromised tissues undergo a complex and ordered series of events to repair the tissue. The series of events may include infiltration of immune cells as part of the process to remove and destroy necrotic tissue, increased vascularization by angiogenic factors and increased cell proliferation and extracellular matrix deposition. Although the basic process of tissue repair has been characterized, the individual steps and factors necessary to carry out this complex series of events are not well understood. The identification of individual steps and factors could lead to improved methods for the treatment of diseases resulting from inadequate wound repair processes.
Previous studies have used the “scratch” wound closure assay to assess the potential effects of an agent on in vitro cell migration. Though informative, such a test does not mimic the dynamic in vivo wound healing conditions to the extent that not all factors involved in wound closure are present in the in vitro assay. For this reason, in vivo systems have been developed to assess the ability of an agent or factor to modulate wound healing activities.
Using these types of in vitro models, a number of specific growth factors have been recognized for their effect on angiogenesis. One such growth factor is TGF-β. This family of dimeric proteins includes TGF-β1, TGF-β2, TGF-β3, TGF-β4, and TGF-β5 which regulate the growth and differentiation of many cell types. This family of proteins exhibits a range of biological effects from stimulating the growth of some cell types (Noda et al., (1989) Endocrinology, 124:2991-2995) and inhibiting the growth of other cell types (Goey et al., (1989) J. Immunol., 143:877-880; Pietenpol et al., (1990) Proc. Nat'l. Acad. Sci. USA, 87:3758-3762). TGF-β has also been shown to increase the expression of extracellular matrix proteins, including collagen and fibronectin (Ignotz et al., (1986) J. Biol. Chem., 261:4337-4345) and accelerates the healing of wounds (Mustoe et al., (1987) Science, 237:1333-1335).
Another growth factor recognized for its effect on angiogenesis is Platelet Derived Growth Factor (PDGF). PDGF was originally found to be a potent mitogen for mesenchymal derived cells (Ross R. et al. (1974) Proc Nat'l Acad Sci USA 71(4):1207-1210.; Kohler N. et al. (1974) Exp. Cell Res. 87:297-301). Further studies have shown that PDGF increases the rate of cellularity and granulation in tissue formation. Wounds treated with PDGF have the appearance of an early stage inflammatory response, including an increase in neutrophils and macrophage cell types at the wound site. These wounds also show enhanced fibroblast function (Pierce, GF et al. (1988) J. Exp. Med. 167:974-987). Both PDGF and TGFβ have been shown to increase collagen formation, DNA content, and protein levels in animal studies. (Grotendorst, GR et al. (1985) J. Clin. Invest. 76:2323-2329.; Sporn, MB et al. (1983) Science 219:1329). The effect of PDGF in wound healing has been shown to be effective in human wounds. In human wounds, PDGF-AA expression is increased within pressure ulcers undergoing healing. The increase of PDGF-AA corresponds to an increase in activated fibroblasts, extracellular matrix deposition, and active vascularization of the wound. Furthermore, such an increase in PDGF-AA is not seen in chronic non-healing wounds. A number of other growth factors having the ability to induce angiogenesis and wound healing include, Vascular Endothelial Growth Factor (VEGF), Keratinocyte Growth Factor (KGF) and basic Fibroblast Growth Factor (bFGF).
However, most of these growth and angiogenic factors have side effects. Accordingly, there is a need for additional factors useful in promoting wound repair.